The invention relates to carburetor for internal combustion engines of the kind which comprises operator actuatable main throttle means located in the induction passage, downstream of the delivery opening of a main fuel delivery system, and an auxiliary starting device for delivering an additional flow of fuel and air to the induction passage during starting and cold running, said starting device comprising duct means opening into the induction passage downstream of the throttle means receiving fuel and air, and provided with a closure valve, temperature sensitive means for maintaining said closure valve in open condition until the temperature of the engine has reached a first predetermined value, a flap located in said induction passage upstream of the opening of the main fuel delivery system, said means being arranged to be biased toward opening by the air flow around it, return means for resiliently biasing the flap toward closure, with a force which decreases as the temperature of the engine increases.
It has already been proposed (French Pat. No. 2 180 160 and U.S. Pat. No. 3,934,571) to combine a closure or distributing valve and a choke flap in a starting device. The flap is placed in the induction passage upstream of the opening of the main delivery system and is unbalanced so that it tends to open under the action of the air stream through the induction passage against the action of return means whose effect on the flap decreases in proportion as the engine warms up. The effect of the flap is to increase the depression or vacuum at the opening of the delivery system and consequently to increase the richness of the air/fuel mixture admitted to the engine, as long as the engine has not reached a predetermined temperature. While that flap provides considerable enrichment when the engine is loaded by opening the main throttle means, it has the disadvantage of lack of accuracy as regards the richness of the mixture supplied to the engine. Furthermore, a starting device combining a distributing valve and a choke valve is complex since it requires elements of both a conventional choke flap and a distributing circuit.
It is an object of the invention to provide a carburetor comprising a starting device which supplies the engine with an appropriate amount of air and fuel under all starting conditions, while it remains relatively simple.
According to an aspect of the invention, there is provided a carburetor of the above-defined kind, wherein the flap is associated with means preventing it from closing beyond a predetermined partially open position, and the closure valve is associated with means which reduce the quantity of fuel flowing in the duct as soon as the engine runs by itself.
According to another aspect, the flap is associated with temperature responsive means preventing it from closing beyond a minimum position variable between a predetermined partially open position when the engine is cold and a fully open position when the engine temperature exceeds a second value. Thus the only function of the flap (in contrast to prior art flaps) is to enrich the mixture supplied to the cold engine, but only when the engine is under load. It can be seen that:
During cranking to start the engine, the distributing valve disposed in a duct receiving a sufficient supply of air and fuel provides the required high richness.
During idling of the engine when cold (running without load), the richness is also determined only by the distributing valve.
When the cold engine is loaded by opening the main throttle means, enrichment is provided both by the distributing valve and by the flap (which ceases to have an appreciable effect at a temperature at which the distributing valve is usually not completely closed).
Finally, when the engine is at its normal operating temperature, the starting device ceases to act on the richness of the mixture.
The flap is typically biased in the closing direction by a temperature sensitive element, subjected to the engine temperature, which may be the same as that controlling the distributing valve. Advantageously the valve is placed in a duct supplied with an air-fuel mixture having a richness which first decreases as soon as the engine is self operating and further decreases after a time period (which may be approximately constant or vary inversely with ambient temperature) after the engine starts.